The performance of certain optical devices strongly depends on the ability of avoiding and/or eliminating stray light. Stray light reduction is an important issue especially in space-borne astronomical instruments, where it may notably affect both the geometric and the radiometric image quality. To prevent light originating from outside the desired field of view of the instrument, baffles are typically arranged around the optical axis. Such baffles are usually cylindrical or conical and may comprise vanes on their interior walls in order to trap as much stray light as possible. In such systems, the absorptance of undesired off-axis radiation in the spectral range of the detector should be as close as possible to 100% irrespective of the incidence angle. Furthermore, the reflectance of the surfaces should ideally be Lambertian. Black coatings have been developed to cover all mechanical surfaces close to the optical beam. An overview can be found in: M. J. Persky, «Review of black surfaces for space-borne infrared systems», Review of scientific instruments, vol. 70, no 5, p. 2193-2217, 1999. Historically, in most spatial missions, black surfaces were obtained using paints (e.g. Aeroglaze™ from Lord Corporation, DeSoto™ Flat Black from Pacific Western Paints, etc.) or anodizations (e.g. Martin Black™, Enhanced Martin Black™, or Infrablack™ from Martin Marietta Corporation, etc.). Other advanced optically black diffuse surfaces such as plasma sprayed boron-on-beryllium, plasma sprayed boron carbide-on-silicon carbide and plasma sprayed beryllium-on-beryllium have been developed. These are diffuse absorptive surfaces that employ microscopic structures to absorb, scatter or trap light. Other black surfaces can be obtained by electrodeposition (e.g. black chrome, black cobalt) or by electroless nickel coating. In the 21st century, several advanced coatings have been developed by various companies. Acktar's inorganic coatings (Nano Black™, Magic Black™, Vacuum Black™, Fractal Black™ and Ultra Black™) are fabricated using vacuum deposition technology and feature very low reflectance, high thermal stability, excellent adhesion, and low outgassing. Surrey NanoSystems have developed a super-black material (called Vantablack™) that absorbs 99.96% of surface light. Vantablack™ is produced using a low-temperature carbon nanotube (CNT) growth process. When light strikes the layer of CNTs, instead of bouncing off it is trapped between the tubes before eventually becoming heat. Titanium and silicon substrates have been used to demonstrate the efficiency.
Apart from optical instruments, black materials, in particular black coatings, have applications in passive thermal management (requiring high emissivity), solar energy harvesting (e.g. solar water heating, concentrated solar power generation, etc.), infrared sensing (e.g. in MEMS IR sensors), thermal actuation (e.g. in MEMS thermal actuators), etc.
It is an object of an aspect of the present invention to provide a ceramic composite that may serve as a black coating. More generally, however, it is an object of the invention to provide a new type of coating.
Aspects of the methods used in the context of the present invention have been developed from earlier works in which the inventor contributed. The interested reader is referred to (1) Bahlawane N, Premkumar P A, Onwuka K, Reiss G, Kohse-Hoinghaus K. Self-catalyzed chemical vapor deposition method for the growth of device-quality metal thin films. Microelectronic Engineering. 2007; 84(11):2481-2485; (2) Bahlawane N, Premkumar P A, Onwuka K, Rott K, Reiss G, Kohse-Hoinghaus K. Catalytically enhanced H2-free CVD of transition metals using commercially available precursors. Surface & Coatings Technology. 2007; 201(22-23):8914-8918; (3) Premkumar P A, Bahlawane N, Kohse-Hoinghaus K. CVD of metals using alcohols and metal acetylacetonates, Part I: Optimization of process parameters and electrical characterization of synthesized films. Chemical Vapor Deposition. 2007; 13(5):219-226; (4) Premkumar P A, Bahlawane N, Reiss G, Kohse-Hoinghaus K. CVD of metals using alcohols and metal acetylacetonates, Part II: Role of solvent and characterization of metal films made by pulsed spray evaporation CVD. Chemical Vapor Deposition. 2007; 13(5):227-231; (5) Premkumar P A, Turchanin A, Bahlawane N. Effect of solvent on the growth of Co and Co2C using pulsed-spray evaporation chemical vapor deposition. Chemistry of Materials. 2007; 19(25):6206-6211; and (6) German patent application DE 10 2006 033 037 A1, disclosing a one-step method for depositing a metal onto a substrate by means of a gas phase deposition method. According to DE 10 2006 033 037 A1, a metal-containing precursor compound is dissolved in an organic solvent, which serves as a reducing agent that releases the metal species from the metalorganic precursor.
General Description
According to a first aspect of the invention, a ceramic composite coating is presented. The ceramic composite coating comprises a ceramic matrix, which is not a carbide matrix, having embedded therein carbide nanoparticles (in particular metal carbide nanoparticles) and/or metal-carbon composite nanoparticles (with separate metal and carbon phases) embedded therein. The ceramic composite coating may be referred to as an “advanced cermet”, since it corresponds to a carbon-pigmented cermet.
The carbide nanoparticles embedded within the ceramic matrix are metastable and the metal-carbon composite nanoparticles are the decay products of the metastable carbide nanoparticles. It is worthwhile noting that the term “metastable” is used herein with its ordinary meaning, i.e. designating a state in which a system may remain for an extended time (when no energy is introduced into the system from the outside), which state is not, however, the system's state of least energy. In the system under consideration, the state of least energy corresponds to the configuration in which a carbon phase and a metal phase coexist as a nanoparticular inclusion within the ceramic matrix. The metastable state (the carbide phase) corresponds to a local minimum of the internal energy of the system, whereas the stable state (the carbon and metal phases) corresponds to the global minimum of the internal energy at room temperature (20° C.) and atmospheric pressure (1013.25 hPa).
The ceramic composite may comprise the metal carbide nanoparticles and the metal-carbon composite nanoparticles in any proportion relative to each other, ranging from 100% carbide nanoparticles and 0% metal-carbon composite nanoparticles to 0% carbide nanoparticles and 100% metal-carbon composite nanoparticles. A ceramic composite with only metal-carbon composite nanoparticles may be obtained by annealing.
The nanoparticles may have an average size (greatest diameter) in the range from 5 to 500 nm, more preferably in the range from 10 to 400 nm, even more preferably in the range from 20 to 300 nm, still more preferably in the range from 20 to 200 nm and most preferably in the range from 20 to 100 nm.
According to a preferred embodiment of the invention, the metal-carbon composite nanoparticles comprise metal cores with carbon shells.
The ceramic matrix may be a boride matrix, a silicide matrix or a nitride matrix. More preferably, however, the ceramic matrix is a metal oxide matrix. Such metal oxide matrix may e.g. consist of an oxide selected from the group consisting of VO2, Al2O3, SiO2, MgO, TiO2, ZrO2, Mn3O4, SnO2, ZnO, spinel having the general formula AB2O3 with A and B being metal cations having different valences, perovskite having the general formula A′B′O3 with A′ and B′ being differently sized metal cations, or mixtures thereof.
According to preferred embodiments of the first aspect of the invention, the carbide nanoparticles consist of carbides of metals selected from the group consisting of Ni, Co, Fe, Cr, Mo, Pt, Pd and mixtures thereof.
The density of the carbide nanoparticles and/or metal-carbon composite nanoparticles in the matrix may be uniform. In some embodiments, however, it may be preferred that the density of the carbide nanoparticles and/or metal-carbon composite nanoparticles in the matrix is non-uniform across the thickness of the ceramic composite. The density may have a constant or a non-constant gradient in the thickness direction.
The ceramic composite coating is a black coating, preferably a matte black coating, more preferably a superblack coating, comprising a ceramic composite as described herein. In the context of the present document, the term “black” qualifies a surface with a total hemispherical reflectivity (THR) of no more than 5% over the entire wavelength range from 400 nm to 1 μm and for any incidence angle (angle between the surface and the incoming beam) greater than 20°. A “matte” surface is a surface, whose reflectivity in the specular direction amounts to no more than 5% of the THR, for any incidence angle greater than 20°. As used herein, a “superblack” surface is a matte black surface having, within the wavelength range from 400 nm to 2.5 μm, a total hemispherical reflectivity (THR) of no more than 1% around normal incidence (incidence angle ≥20°) and no more than 10% for grazing incidence (incidence angle <20°).
As will be appreciated, a black coating consisting of the advanced cermet of the present invention offers good absorptance and can be tuned to meet the requirements of a superblack coating. Furthermore, Lambertian reflection behavior can be achieved. The advanced cermet coating is suitable for heat radiation (emittance) and can thus be used in thermal elements and on baffles, vanes or optical elements that need that type of cooling. The advanced cermet is compatible with different kinds of substrates, it can be produced with any desired thickness in the range of 30 nm to 1000 μm. The surface density (mass per unit area) is compatible with most applications. As the fabrication process of the advanced cermet uses chemical vapor deposition (CVD), which involves deposition at elevated temperatures and which may be carried out at low pressures, conditions that are unfavourable for the incorporation of volatile organic molecules or water into the coating, outgassing is not an important issue with advanced cermets. That point is especially advantageous for space and high-vacuum applications. Another advantage of advanced cermets is their ability to withstand direct sun illumination or, more generally speaking, intense radiation without alteration. Last but not least, the chemical inertness of the advanced cermets is an advantage (e.g. low sensitivity to atomic oxygen) appreciated in many applications.
A further aspect of the present invention relates to a method for producing an advanced cermet as described herein using CVD. The method comprises introducing at least one first precursor for depositing the ceramic matrix into a reaction chamber, introducing second precursors for depositing the carbide nanoparticles into the reaction chamber, the second precursors comprising an inorganic, metalorganic or organometallic precursor and at least one of an alcohol and an aldehyde, transporting the precursors to a substrate maintained at the deposition temperature, and forming the ceramic matrix from the at least one first precursor and the embedded carbide nanoparticles from the second precursors.
As used herein, the terms “first precursor” or “first precursors” refer to the precursor or group of precursors from which the ceramic matrix is deposited. The term “second precursors” designates the group of precursors from which the carbide nanoparticles are deposited. The numerals “first” and “second” are used for distinguishing between these precursors or precursor groups; no implication regarding the order or the importance, the quantity etc. of the precursors is thereby intended.
CVD and its different formats, e.g. metalorganic CVD (MOCVD), atomic layer deposition (ALD), pulsed-spray evaporation CVD, etc. are proposed for the production of the advanced cermet coating. These high technological gas-phase processes enable the growth of uniform films even on highly structured surfaces. Gradient carbon-pigmented metal-metal oxide nanocomposite coatings can be achieved. The overall structure features a metal oxide matrix thin film in which particles with controlled size, density and composition are embedded. The process parameters can easily be tuned such that the resulting structures satisfy adhesion and durability criteria. Furthermore, no issues are expected with particulate contamination, outgassing, water uptake and cleanability. The nanoparticles' loading in the ceramic matrix may be performed in a decreasing manner along the deposition process in order to gradually reduce the refractive index and avoid light reflection at the surface. The metal-carbon nanoparticles strongly absorb visible light because of the inter-band transition of the metal phase and the intrinsic absorption bands of the carbon phase. The scattering and absorption efficiencies can be manipulated through the adjustment of the size of the particles and the proportion of carbon to metal. Compared with ordinary cermets, the carbon phases of the nanoparticles lead to an improvement of the absorption behaviour and prevent the coating from overheating.
The proposed fabrication method may involve only moderate heating of the substrate (e.g. up to 350° C. or 400° C.) and thereby offers a great range of possibilities regarding the choice of the substrate. Aluminium parts or other metallic parts in a precise metallurgical state (that shall not be altered) could thus serve as substrates. Other possible substrates are silicon, glass, etc.
Another noteworthy advantage of the proposed method is that it is not a so-called line-of-sight process (where shadowing is a concern) and complex geometries may thus be coated. Complex three-dimensional parts (e.g. baffles with vanes, etc.) can thus be coated much more easily than in those processes.
A considerable degree of freedom is available to optimize the optical properties of the advanced cermet, including:                Thickness        Composition of the matrix (choice of the ceramic)        Nature of the involved metal        Size, density and density profile of the nanoparticles within the film        Fraction of carbon to metal        
According to a preferred embodiment of the method, the at least one first precursor and the second precursors are introduced into the reaction chamber at respective times, the reaction chamber being purged there between (e.g. by using a chemically inert gas like N2 or the like), the introductions of the at least one first precursor and the second precursors being repeated plural times. The number of cycles may depend on several parameters, in particular the desired thickness, the duration of exposure of the substrate to each of the first and second precursors, the type of materials involved, etc.
The at least one first precursor and/or the second precursors may be inorganic precursors, such as, e.g., halides, carbonyls, nitrates, etc. Preferably, however, the at least one first precursor and/or the second precursors comprise metalorganic or organometallic compounds; still more preferably metal alkoxides or metal β-diketonates. Metalorganic or organometallic precursors are, preferred, as they are typically less toxic and corrosive than inorganic precursors and have lesser demands regarding recovery and disposal of reaction products.
The ceramic matrix formed from the first precursor is preferably a metal oxide matrix consisting of VO2, Al2O3, SiO2, MgO, TiO2, ZrO2, Mn3O4, SnO2, ZnO, spinel having the general formula AB2O3 with A and B being metal cations having different valences, perovskite having the general formula A′B′O3 with A′ and B′ being differently sized metal cations, or mixtures thereof.
The carbide nanoparticles formed from the second precursors preferably consist of carbides of metals selected from the group consisting of Ni, Co, Fe, Cr, Mo, Pt, Pd and mixtures thereof.
The method may comprise annealing the ceramic matrix with the embedded carbide nanoparticles so as to convert at least part of the carbide nanoparticles into metal-carbon composite nanoparticles.
Preferably, the CVD is pulsed spray evaporation CVD, the at least one first precursor being injected into the reaction chamber as a first precursor solution and the second precursors being injected into the reaction chamber as a second precursor solution. In that case, the first precursor solution preferably comprises vanadium oxy-tri-isopropoxide dissolved in an organic solvent (e.g. an alcohol) and the second precursor solution preferably comprises cobalt acetylacetonate and/or nickel acetylacetonate dissolved in alcohol.
It is one noteworthy advantage of the method that it does not require the presence of strong reducing agents for the metal components.
The deposition temperature is preferably not higher than 700° C., more preferably not higher than 650° C., more preferably not higher than 600° C. Preferably, the deposition temperature is selected in the range from 250° C. and 700° C., more preferably in the range from 300° C. to 650° C., still more preferably in the range from 400° C. to 650° C., even still more preferably in the range from 450° C. to 600° C.
In case of pulsed spray evaporation CVD, the rate of the injections of the first and/or the second precursors is preferably comprised in the range from 0.5 to 100 Hz, more preferably in the range from 1 to 50 Hz, even more preferably in the range from 1 to 25 Hz, still more preferably in the range from 1 to 10 Hz and most preferably in the range from 2 to 10 Hz. Still in case of pulsed spray evaporation CVD, the temperature of evaporation and/or transport is comprised in the range from 100° C. to 300° C., more preferably in the range from 150° C. to 250° C.